In order to send materials into space to fabricate a structure, the volume of each material must be kept as small as possible for transportation. For example, a large apparatus such as a panel for a solar cell to be installed on a satellite or space structure is required to be reduced in size when transporting from the Earth. An apparatus folded when transporting is expanded into a predetermined shape suitable for use in space such as on a satellite orbit. The property of a material which permits shrinking for transport and expansion or deployment into a predetermined shape upon practical use is called “inflatability”.
Inflatability is also an important property for materials used for ground-based structures. Use of an inflatable material enables compact storage (volume reduction) when loading on a delivery vehicle or the like and expansion into a predetermined shape upon use at a fabrication or building site.
Structures having inflatability can be classified into two categories: structures which make use of mechanical action such as folding at joints, and those which make use of action derived from a material property, that is, restoration into the original shape by heating.
Conventional inflatability tended to result from mechanical actions and folding at a joint part is often used. Upon use, therefore, some power has to be applied to the joint part to expand the structure into a predetermined shape. Such a structure has the drawback that problems such as breakdowns or accidents may occur upon expansion.
Several studies have so far been made on structures having inflatability which depends on the action of a material property. When used to construct a large-sized structure, a material having inflatability must have certain strength by having it go through a process of making it more rigid. Preferred examples of a polymer material which can be rigidified and provide sufficient strength include fiber reinforced plastic (FRP) and carbon fiber reinforced plastic (CFRP). In order to let FRP have inflatability, fibrous material may be insided in a shape memory polymer material.
The term “shape memory polymer” as used herein means, among conventional polymers, a resin whose molded shape and deformed shape can be used selectively by temperature operation with heat. A molded product of a shape memory polymer using such a resin is deformed at a glass transition point of the polymer or above but at a temperature less than a molding temperature, and then cooling it to the glass transition point (Tg) or below while keeping the deformed shape, thereby fixing the deformed shape; and restoring the molded shape by heating it at a temperature of the glass transition point or above but at a temperature less than the molding temperature. Thus, the deformed and molded shapes can be used selectively by temperature operation.
The term “FRP” generally means fiber reinforced plastic containing a continuous fibrous material. It has hardness comparable to ceramic and strength comparable to a metal. It weighs about one fifth of iron, but has a modulus of elasticity about 3 to 4 times superior to that of iron. It has been an important theme to find how dense fibers and resin can be packed in FRP, particularly CFRP, per unit cross-sectional area thereof.
The proportion of fibers determines the strength of FRP. With an increase in the proportion of fibers, the strength of FRP increases. Upon molding such FRP into a plate or the like, resin is necessary for the purpose of fiber-to-fiber adhesion. There are various weaving or twisting types of these fibers used for FRP. For example, a cloth having even a width of about 10 m is usable and it is suitable for use in a plate or pipe of large-sized structures.
When such FRP is produced, a fibrous material must be immersed with a resin. The material can be immersed better with a thermosetting resin than with a thermoplastic resin. Since the thermosetting resin usually has a low viscosity, it enables production of fine FRP and the resulting FRP is capable of maintaining high strength. The thermoplastic resin on the other hand softens easily, leading to a difficulty in retaining the structure of FRP.
However, use of a thermosetting resin having a shape memory property for the production of FRP has conventionally involved the drawback that a two-part curable resin, for example, cures immediately after mixing and time (pot life) necessary for operation such as impregnation cannot be secured.